This application is based on and hereby claims priority to European Application No. 991096272 filed on May 14, 2001, and PCT Application No. PCT/EP00/04319 filed on May 12, 2000, the contents of which are hereby incorporated by reference.
The invention relates to a component, in particular a component which can be exposed to hot vapor, having a metallic base body which has a protective coating in order to increase the resistance of the base material to oxidation. The invention also relates to a process for producing a protective coating in order to increase the resistance to oxidation on a component which can be exposed to hot vapor, having a metallic base body which has a base material.
In various technical fields, components are exposed to hot vapor, in particular steam. This applies, for example, to components used in steam installations, in particular in steam power plants. With a view to increasing the efficiency of steam power plants, the efficiency is increased, inter alia, by raising the steam parameters (pressure and temperature). Future developments will involve pressures of up to 300 bar and temperatures of up to over 650xc2x0 C. To produce elevated steam parameters of this level, there is a need for suitable materials with a high creep strength at elevated temperatures.
Since austenitic steels, on account of unfavorable physical properties, such as a high coefficient of thermal expansion and low thermal conductivity, in this case meet their limits, numerous variants of ferritic-martensitic steels with a high creep strength and chromium contents of from 9% by weight to 12% by weight are currently being developed.
EP 0 379 699 A1 has disclosed a process for increasing the resistance of a blade of a thermal machine, in particular a blade of an axial compressor, to corrosion and oxidation.
The base material of the compressor blade in this case is formed of a ferritic-martensitic material. A securely adhered surface-protection layer comprising 6 to 15% by weight of silicon, remainder aluminum, is sprayed onto the base material using the high-speed method with a particle velocity of at least 300 m/s onto the surface of the base material. A conventional paint-spraying process is used to apply a plastic, for example polytetrafluoroethylene, to this metal protective layer, which plastic forms the covering layer (outer layer) of the blade. The process provides a protective layer on a blade which has an increased resistance to corrosion and erosion in the presence of steam and at relatively moderate temperatures (450xc2x0 C.), as are relevant to compressor blades.
The article xe2x80x9cWerkstoffkonzept fxc3xcr hochbeanspruchte Dampfturbinen-Bauteilexe2x80x9d, by Christina Berger and Jxc3xcrgen Ewald in Siemens Power Journal April 1994, pp. 14-21, has provided an analysis of the materials properties of forged and cast chromium steels. The creep strength of chromium steels containing 2 to 12% by weight of chromium and additions of molybdenum, tungsten, niobium and vanadium decreases continuously as the temperature rises. For use at temperatures of over 550 to 600xc2x0 C., forged shafts are described, which contain from 10 to 12% by weight of chromium, 1% of molybdenum, 0.5 to 0.75% by weight of nickel, 0.2 to 0.3% by weight of vanadium, 0.12 to 0.23% by weight of carbon and optionally 1% by weight of tungsten. Castings produced from chromium steel are used in valves for a steam turbine, outer and inner casings of high-pressure, medium-pressure, low-pressure and saturated-steam turbines. For valves and casings which are exposed to temperatures of 550 to 600xc2x0 C., steels which contain 10 to 12% by weight of chromium are used, and these steels may in addition contain 0.12 to 0.22% by weight of carbon, 0.65 to 1% by weight of manganese, 1 to 1.1% by weight of molybdenum, 0.7 to 0.85% by weight of nickel, 0.2 to 0.3% by weight of vanadium or also 0.5 to 1% by weight of tungsten.
The article xe2x80x9cSteam Turbine Materials: High Temperature Forgingsxe2x80x9d by C. Berger et al., 5th Int. Conf. Materials for Advanced Power Engineering, Liege, Belgium, Oct. 3-6, 1994, provides a summary of the development of CrMoV steels which contain from 9 to 12% by weight of chromium and have a high creep strength. These steels are in this case used in steam power installations, such as conventional steam power plants and nuclear power plants. Components produced from chromium steels of this type are, for example, turbine shafts, casings, bolts, turbine blades, pipelines, turbine-wheel disks and pressure vessels. A further summary of the development of new materials, in particular 9-12% by weight chromium steels, is given by the article xe2x80x9cMaterial development for high temperature-stressed components of turbomachinesxe2x80x9d by T. -U. Kern et al. in Stainless Steel World, October 1998, pp. 19-27.
Further application examples for chromium steels containing 9% by weight to 13% by weight of chromium are given, for example, in U.S. Pat. No. 3,767,390. The martensitic steel used in this document is employed for steam-turbine blades and the bolts which hold together the casing halves of a steam turbine.
EP 0 639 691 A1 has disclosed a turbine shaft for a steam turbine which contains 8 to 13% by weight of chromium, 0.05 to 0.3% by weight of carbon, less than 1% of silicon, less than 1% of manganese, less than 2% of nickel, 0.1 to 0.5% by weight of vanadium, 0.5 to 5% by weight of tungsten, 0.025 to 0.1% by weight of nitrogen, up to 1.5% by weight of molybdenum, and also between 0.03 and 0.25% by weight of niobium or 0.03 and 0.5% by weight of tantalum or less than 3% by weight of rhenium, less than 5% by weight of cobalt, less than 0.05% by weight of boron, with a martensitic structure.
WO 91/08071 relates to a protective layer protecting against corrosive and erosive attack at a temperature of up to approximately 500xc2x0 C. for a substrate formed of a chromium steel. A protective layer which contains aluminum is formed on the substrate. The aluminum-containing protective layer is applied electrochemically, in particular by electrodeposition, and is hardened or age-hardened at least on its surface in order to form the protective layer. As a result, a so-called duplex layer is formed, which comprises the metal layer and the hard layer.
It is an object of one aspect of the invention to provide a component which can be exposed to hot vapor, having a metallic base body, which has an increased resistance to oxidation compared to the metallic base body. A further possible object of the invention is to describe a process for producing a protective coating in order to increase the resistance to oxidation of the base material on a component.
According to one aspect of the invention, the object relating to a component is achieved by the fact that the component has a protective layer, which has a thickness of less than 50 xcexcm and contains aluminum, on the base material.
One aspect of the invention is based on the discovery that, when a base material is used at elevated temperatures, for example in steam power plants, as well as a high creep strength a considerable resistance to oxidation in the steam is also necessary. The oxidation of the base materials in some cases increases considerably as the temperature rises. This oxidation problem is intensified by the reduction in the chromium content of the steels used, since chromium as an alloying element has a positive influence on the resistance to scaling. Therefore, a lower chromium content can increase the rate of scaling. By way of example, in the case of steam generator tubes, thick oxidation layers on the steam side may lead to a deterioration in the heat transfer from the metallic base material to the steam and therefore to the temperature of the pipe wall rising and to the service life of the steam-generator pipes being reduced. In steam turbines, by way of example jamming of screw connections and valves caused by scaling and an additional load caused by the growth of scale in blade grooves, or flaking of scale at blade outlet edges, could lead to an increase in the notch stress.
Because it has an adverse effect on the mechanical properties of the base material, the possibility of the resistance to scaling by changing the alloying composition of the base material using elements which reduce scaling, such as chromium, aluminum and/or silicon, in an increased concentration is ruled out. By contrast, one aspect of the invention, which has a thin aluminum-enriched zone of the base material, already increases the resistance of the base material to oxidation by up to more than one order of magnitude. Furthermore, this allows fully machined components to be protected without problems, by providing them with an oxidation coating of this type. On account of the low thickness of the protective layer, there is also no adverse effect on the mechanical properties of the base material. The protective layer is in this case to a large extent, possibly completely, formed by the diffusion of aluminum into the base material or by the reverse process. Corresponding diffusion of the aluminum into the base material and of elements of the base material into an aluminum layer may take place as part of a heat treatment carried out at below the tempering temperature of the base material, so that there is no need for a further heat treatment of the component. If appropriate, diffusion of this type may also take place when the component is being used at the prevailing temperatures. A high adhesive strength is achieved as a result of the metallic bonding between the aluminum and the alloying elements of the base material. Moreover, the protective layer has a high hardness, so that it is also highly resistant to abrasion. Furthermore, it is also possible to achieve a particularly uniform formation of the layer thickness of the protective layer even at locations which are difficult to gain access to, on account of simple application methods being used.
The thickness of the protective layer is preferably less than 20 xcexcm, in particular less than 10 xcexcm. It may preferably be between 5 and 10 xcexcm.
The proportion of aluminum in the protective layer is preferably over 50% by weight.
The protective layer preferably contains, in addition to aluminum, iron and chromium, which may, for example, have diffused into the protective layer from a base material or have been applied to the base material, together with an aluminum-containing layer. Furthermore, the protective layer may, in addition to aluminum, also contain silicon, in particular in a proportion of up to 20% by weight. Suitable addition of silicon enables the hardness of the protective layer, as well as other mechanical properties, to be set as desired.
The base material of the component is preferably a chromium steel. It may contain between 0.5% by weight and 2.5% by weight of chromium, and also between 8% by weight and 12% by weight of chromium, in particular between 9% by weight and approximately 10% by weight of chromium. As well as chromium, a chromium steel of this type may also contain between 0.1 and 1.0, preferably 0.45% by weight of manganese. It may also contain carbon in a proportion of between 0.05 and 0.25% by weight, silicone in a proportion of less than 0.6% by weight, preferably approximately 0.1% by weight, molybdenum in a proportion of between 0.5 and 2% by weight, preferably approximately 1% by weight; nickel in a proportion of up to 1.5% by weight, preferably 0.74% by weight; vanadium in a proportion of between 0.1 and 0.5% by weight, preferably approximately 0.18% by weight; tungsten in a proportion of between 0.5 and 2% by weight, preferably 0.8% by weight; niobium in a proportion of up to 0.5% by weight, preferably approximately 0.045% by weight; nitrogen in a proportion of less than 0.1% by weight, preferably approximately 0.05% by weight, and if appropriate an addition of boron in a proportion of less than 0.1% by weight, preferably approximately 0.05% by weight.
The base material is preferably martensitic or ferritic-martensitic or ferritic.
The component which has the thin protective layer is preferably a component of a steam turbine or a component of a steam generator, in particular a steam-generator pipe. The component may be a forging or a casting. A component of a steam turbine may in this case be a turbine blade, a valve, a turbine shaft, a wheel disk of a turbine shaft, a connecting element, such as a screw, a bolt, a nut, etc., a casing component (inner casing, guide-vane support, outer casing), a pipeline or the like.
The object relating to a process for producing a protective coating for increasing the resistance to oxidation on a component which can be exposed to hot vapor may be achieved by the fact that a layer which is less than 50 xcexcm thick and contains aluminum pigment is applied to a metallic base body, which has a base material, and the component is held at a temperature which is lower than the tempering temperature of the base material, so that a reaction takes place between the aluminum and the base material in order to form an aluminum-containing protective layer.
The aluminum-containing layer is in this case preferably held at a temperature in the region of the melting temperature of aluminum, in particular between 650xc2x0 C. and 720xc2x0 C., in order to carry out the diffusion. The temperature may also be lower. If appropriate, the diffusion may also take place while the component is being used in a steam plant at the prevailing temperature of use. The component is exposed to the appropriate temperature for carrying out the reaction for at least 5 min, preferably over 15 min, if appropriate even for a few hours.
The layer containing the aluminum is preferably applied in a thickness, in particular a mean thickness, of between 5 xcexcm and 30 xcexcm, in particular between 10 xcexcm and 20 xcexcm. The thin layer containing aluminum pigment is, for example, applied by an inorganic high-temperature coating. The layer may be applied by being sprayed on, with the result that a suitable protective coating of the component can be achieved even at locations which are difficult to gain access to. A heat treatment of the component in order to carry out the reaction between base material and coating can take place, for example, in the furnace or by using other suitable heat sources.
After the heat treatment of the applied layer containing aluminum pigment has been carried out, a substantially continuous protective layer, which is approx. 5 to 10 xcexcm thick and contains Fexe2x80x94Alxe2x80x94Cr, can be formed, i.e. in the form of an intermetallic compound between aluminum and the base material. The application of the layer to a chromium steel leads to a considerable improvement of the scaling behavior of the base material. On account of a high aluminum content, in particular of over 50% by weight, in the protective layer which is formed as a result of reaction between the aluminum pigments and the base material, in particular a diffusion layer, the resistance of the component to oxidation is considerably increased. The protective layer formed in this way has a high hardness (Vickers Hardness HV) of, for example, approximately 1200.
Alternatively, the application of a thin aluminum-containing layer of this type may also take place by an adapted dip-aluminizing process. The change in the dip-aluminizing process is carried out in such a way that, compared to the standard aluminum-containing layer thicknesses of between 20 and 400 xcexcm, the layer thickness is reduced accordingly. Aluminum hot-dip layers produced by the hot-dip process form a plurality of phases (Eta phase/Fe2Al5; Zeta phase/FeAl2, Theta phase/FeAl3) with iron. In the conventional hot-dipping (hot-dip aluminizing) for simple steel parts, suitably pretreated components which are to be coated are immersed in molten aluminum or aluminum alloy baths at temperatures of from 650xc2x0 C. to 800xc2x0 C. and are pulled out again after a residence time of 5 to 60 sec. In the process, an intermetallic protective layer and, on this, an aluminum covering layer are formed. These coatings which are produced by conventional hot-dip aluminizing present the risk, however, that the top aluminum covering layers introduce aluminum into the steam cycle as a result of the action of steam, which could cause undesirable accompanying phenomena, such as relatively insoluble aluminum silicate deposits.